2018-07-30 18:35 GMT+03:00 Laurent Pinchart <laurent.pinchart@xxxxxxxxxxxxxxxx>: > Hi Matwey, > > On Tuesday, 24 July 2018 21:56:09 EEST Matwey V. Kornilov wrote: >> 2018-07-23 21:57 GMT+03:00 Alan Stern: >> > On Mon, 23 Jul 2018, Matwey V. Kornilov wrote: >> >> I've tried to strategies: >> >> >> >> 1) Use dma_unmap and dma_map inside the handler (I suppose this is >> >> similar to how USB core does when there is no URB_NO_TRANSFER_DMA_MAP) >> > >> > Yes. >> > >> >> 2) Use sync_cpu and sync_device inside the handler (and dma_map only >> >> once at memory allocation) >> >> >> >> It is interesting that dma_unmap/dma_map pair leads to the lower >> >> overhead (+1us) than sync_cpu/sync_device (+2us) at x86_64 platform. >> >> At armv7l platform using dma_unmap/dma_map leads to ~50 usec in the >> >> handler, and sync_cpu/sync_device - ~65 usec. >> >> >> >> However, I am not sure is it mandatory to call >> >> dma_sync_single_for_device for FROM_DEVICE direction? >> > >> > According to Documentation/DMA-API-HOWTO.txt, the CPU should not write >> > to a DMA_FROM_DEVICE-mapped area, so dma_sync_single_for_device() is >> > not needed. >> >> Well, I measured the following at armv7l. The handler execution time >> (URB_NO_TRANSFER_DMA_MAP is used for all cases): >> >> 1) coherent DMA: ~3000 usec (pwc is not functional) >> 2) explicit dma_unmap and dma_map in the handler: ~52 usec >> 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~56 >> usec > > I really don't understand why the sync option is slower. Could you please > investigate ? Before doing anything we need to make sure we have a full > understanding of the problem. Hi, I've found one drawback in my measurements. I forgot to fix CPU frequency at lowest state 300MHz. Now, I remeasured 2) dma_unmap and dma_map in the handler: 2A) dma_unmap_single call: 28.8 +- 1.5 usec 2B) memcpy and the rest: 58 +- 6 usec 2C) dma_map_single call: 22 +- 2 usec Total: 110 +- 7 usec 3) dma_sync_single_for_cpu 3A) dma_sync_single_for_cpu call: 29.4 +- 1.7 usec 3B) memcpy and the rest: 59 +- 6 usec 3C) noop (trace events overhead): 5 +- 2 usec Total: 93 +- 7 usec So, now we see that 2A and 3A (as well as 2B and 3B) agree good within error ranges. > >> So, I suppose that unfortunately Tomasz suggestion doesn't work. There >> is no performance improvement when dma_sync_single is used. >> >> At x86_64 the following happens: >> >> 1) coherent DMA: ~2 usec > > What do you mean by coherent DMA for x86_64 ? Is that usb_alloc_coherent() ? > Could you trace it to see how memory is allocated exactly, and how it's mapped > to the CPU ? I suspect that it will end up in dma_direct_alloc() but I'd like > a confirmation. usb_alloc_coherents() ends up inside hcd_buffer_alloc() where dma_alloc_coherent() is called. Keep in mind, that requested size is 9560 in our case and pool is not used. > >> 2) explicit dma_unmap and dma_map in the handler: ~3.5 usec >> 3) explicit dma_sync_single_for_cpu (no dma_sync_single_for_device): ~4 usec >> >> So, whats to do next? Personally, I think that DMA streaming API >> introduces not so great overhead. > > It might not be very large, but with USB3 cameras at high resolutions and > framerates, it might still become noticeable. I wouldn't degrade performances > on x86, especially if we can decide which option to use based on the platform > (or perhaps even better based on Kconfig options such as DMA_NONCOHERENT). PWC is discontinued chip, so there will not be any new USB3 cameras. Kconfig won't work here, as I said before, DMA config is stored inside device tree blob on ARM architecture. > >> Does anybody happy with turning to streaming DMA or I'll introduce >> module-level switch as Ezequiel suggested? > > A module-level switch isn't a good idea, it will just confuse users. We need > to establish a strategy and come up with a good heuristic that can be applied > at compile and/or runtime to automatically decide how to allocate buffers. I am agree in general, but I cannot understand why webcam driver should think about memory allocation heuristics. > > -- > Regards, > > Laurent Pinchart > > > -- With best regards, Matwey V. Kornilov. Sternberg Astronomical Institute, Lomonosov Moscow State University, Russia 119234, Moscow, Universitetsky pr-k 13, +7 (495) 9392382
